He was smeared with false antisemitism claims, hence the massive defeat. One of those involved in the smearing was... Starmer.
Corbyn was never going to be "allowed" to be Prime Minister. Also, listen to his recent interview where he says he was asked by a committee if he would guarantee to be 100% behind any military action instigated by Israel.
> An investigation based on the largest leak of documents in British political history. The Labour Files examines thousands of internal documents, emails and social media messages to reveal how senior officials in one of the two parties of government in the UK ran a coup by stealth against the elected leader of the party.
Politician smeared during election? Shock horror, it happens during every election.
Leader of the party can't unite their own party so there is a plan to oust them? That's politics.
Jeremy couldn't particularly unite the party, didn't take the center ground, and while I don't think he was a true antisemite there were enough mis-steps there that it meant that the claim could stick (along with the IRA sympathizer claims).